uniform sampler2D uTexture;
varying vec2 vUv;
#define STEP 0.03125

void main() {
  vec4 color = texture2D( uTexture, vUv ); // current value

  // Ground
  if ( vUv.y < STEP ) {
    color.g = 1.0;
  }

  gl_FragColor = color;
}